Catalytic cracking apparatuses are crucial means for crude oil refining, and the economic benefits of refineries depend on the overall product distribution of these apparatuses. Recently, because of the growing trend towards crude oils having higher density and poorer quality, a higher heavy oil conversion capacity and higher selectivity for high-value products are demanded for FCC catalysts. The type Y molecular sieve has been a major provider of the cracking activity of heavy oil catalytic cracking catalysts, and its activity stability and cracking activity are key factors in determining the heavy oil conversion capacity of FCC catalysts.
Accordingly, extensive investigations have been carried out in domestic and abroad research institutions in order to improve the cracking activity and activity stability of type Y molecular sieves. Currently, it is largely agreed that the framework structural stability and the activity stability of molecular sieves can be improved by localizing as many rare earth ions as possible in sodalite cages in the process of rare earth modification of molecular sieves so as to suppress dealumination of the molecular sieve framework during steam aging. Patent ZL200410058089.3 describes a method for preparing rare earth-modified type Y molecular sieves, comprising steps of adjusting the pH of the system to 8-11 using an alkali solution after completion of a rare earth exchange reaction, and then carrying out conventional subsequent treatment processes. In the molecular sieves prepared by this method, rare earth ions are completely located in small cages (sodalite cages). Patent ZL200410058090.6 describes the reaction performance of the molecular sieves of ZL200410058089.3, wherein the catalyst reaction results show that localization of rare earth metals in sodalite cages improves the structural stability and the activity stability of the molecular sieves, manifested in that the heavy oil conversion capacity of the catalyst is greatly improved, although this catalyst has poor coke selectivity.
Chinese patent ZL97122039.5 describes a preparation method for ultra-stable Y zeolites, comprising steps of putting a Y zeolite into contact with an acid solution and an ammonium-containing solution, and subjecting them to a high-temperature steam treatment, wherein the amount of the acid used is 1.5 to 6 moles of hydrogen ions per mole of framework aluminum, the concentration of the acid solution is 0.1 to 5 N, the Y zeolite is kept in contact with the acid solution at a temperature of 5 to 100° C. for a duration of 0.5 to 72 h, and the weight ratio between the Y zeolite and the ammonium ion is 2 to 20. The modification method in accordance with this patent requires addition of an ammonium-containing solution for the purpose of lowering the sodium oxide content in the molecular sieve or reducing the damage to the molecular sieve structure caused by acidic gases during calcination. The FCC catalyst prepared using such molecular sieves is characteristic in its high capacity of heavy oil conversion and a high light-oil yield. However, this modification technique for molecular sieves have the following technical disadvantages: 1) since a large number of ammonium ions are added in the preparation process, ammonium-containing ions eventually enter the atmosphere or waste water, increasing ammonia nitrogen pollution and the cost for pollution control; 2) the method of this patent is unable to solve the issue of particle agglomeration in molecular sieves, which issue reduces specific surface area and pore volume of the molecular sieve and increases the obstruction in the pore channel during exchange in the molecular sieve, making it difficult to accurately and quantitatively localize the modifying element in the cages of the molecular sieve; 3) moreover, in this patent it is further mentioned that rare earth ions may also be introduced by ion exchange, during or after the contact between the Y zeolite and the ammonium-containing solution, and that during the ion exchange, ammonium ions compete with rare earth ions and preferentially take up the positions intended for rare earth ions, thereby hindering rare earth ions from entering the cages of the molecular sieve by exchange, and also lowering the utilization of rare earth ions.
Chinese patent ZL02103909.7 describes a method for preparing rare earth-containing ultra-stable Y molecular sieves by subjecting a NaY molecular sieve to one exchange process and one calcination process, characterized in that the NaY molecular sieve is placed in an ammonium solution and subjected to chemical dealumination at 25 to 100° C. for 0.5 to 5 h, wherein the chemical dealumination chelating agent contains oxalic acid and/or oxalate salts, a rare earth solution is then introduced under stirring to produce a rare earth precipitate that contains rare earth oxalate, and the precipitate is filtered and washed to give a filter cake, followed by a hydrothermal treatment to afford the molecular sieve product. Although the molecular sieve prepared by this method has certain resistance to vanadium contamination, it has relatively low activity stability and cracking activity, and is insufficient to meet the requirement set out by the growing trend towards crude oils having higher density and poorer quality. This issue is mainly attributed to the distribution of rare earth ions in the super-cages and sodalite cages of the molecular sieve during modification. This method demonstrates that rare earth ions are present in the molecular sieve system in two forms, i.e., a part of the rare earth enters sodalite cages in an ionic form while the other part is scattered over the surface of the molecular sieve as an independent phase of rare earth oxide (the precursor of which is rare earth oxalate and is converted into rare earth oxide after subsequent calcination). Such distribution reduces the stabilizing and supporting effect of rare earth ions on the molecular sieve structure. Furthermore, this method also poses a remarkable problem of ammonium nitrogen pollution, and the oxalic acid or oxalate salts added are also toxic and detrimental to the environment and human.
CN200410029875.0 discloses a preparation method for rare earth ultra-stable type Y zeolite, characterized by a step of treating zeolite with a mixed solution comprising a rare earth salt and citric acid or with a mixed solution comprising an inorganic ammonium salt, a rare earth salt and citric acid. This method simplifies the process, and the zeolite prepared thereby, when serving as an active component of a cracking catalyst, is advantageous in lowering the olefin content in gasoline products obtained from the catalytic cracking, and substantially increasing the yield of light oil products obtained from the catalytic cracking. However, this method does not specify the location of rare earth ions in the molecular sieve.